Enable lamp 'Use Nodes' from python Console
This is the python command used to enable Use Nodes and change a lamp strength

Blender 2.79

# enable Use Nodes
bpy.data.lamps["your_light_name"].use_nodes = True/False

# set strength, set the default_value to whatever you like
bpy.data.lamps["your_light_name"].node_tree.nodes["Emission"].inputs[1].default_value = 10    


Blender 2.8

# enable Use Nodes
 bpy.data.lights["your_light_name"].use_nodes = True/False

# set strength, set the default_value to whatever you like
bpy.data.lights["your_light_name"].node_tree.nodes["Emission"].inputs[1].default_value = 10